Nucleophilic substitution of carbon nanotubes

ABSTRACT

Compounds are attached to carbon nanotubes (CNT) by a process which comprises: subjecting surface treated CNTs which have been treated to induce negatively charged surface groups thereon, to nucleophilic substitution reaction with a compound carrying a functional group capable of reacting with the negatively charged groups on the CNT surface, whereby the compound chemically bonds to the CNT. The surface CNT treatment may be reduction. The compounds which are bonded to the CNT may be epoxy resins, bonded directly or through a spacer group. Bi-functional CNTs, grafted to both epoxy resins and other polymers such as polystyrene, are also made by this process.

FIELD OF THE INVENTION

This invention relates to the field of nanotechnology. Moreparticularly, it relates to carbon nanotubes, and to methods ofattaching carbon nanotubes to structural materials such as epoxy resins.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) are hollow carbon architectures made ofconcentric graphene sheets. They exhibit exceptional mechanical,electrical and thermal properties; the best of any known material.Combined with their very high aspect ratios that can reach well over1000, CNT are truly the ultimate additives for the fabrication ofmultifunctional composites. Their diameter is of the order of a fewnanometers, and up to several millimeters in length. Carbon nanotubescan be divided into two general classes: single-walled (SWCNT-only onegraphene sheet) and multi-walled (MWCNT-multiple graphene sheets nestedinto one another). It is generally agreed that for composite work, SWCNTare superior to MWCNT especially if multifunctionality is sought.

Due to the extended sp² hybridization network and their ability to formbundles, CNT are chemically very stable with poor compatibility withpractically any solvents and matrices. To circumvent this problem, CNTneed to be “chemically primed” to facilitate their integration andproduce a good bonding interface. Chemical priming is achieved byanchoring of chemical functions at the surface. This invention refers tomethods to integrate SWCNT in epoxy resins. The methods are applicableto CNTs of all kinds.

BRIEF REFERENCE TO THE PRIOR ART

Attachment of CNT to epoxide containing monomers and other chemicalcompounds including its polymers is a tedious, time consuming and costlyprocess. Neutral CNT cannot be anchored directly to epoxide moieties.Chemical functionalization with suitable functional groups is necessary.

Currently, covalent attachment of CNT to epoxide containing species isdone by first anchoring reactive functional groups such as —NH₂ or —COOHto the external wall of CNT and then subject the functionalized CNT tothe epoxide containing species. The functionalization of CNT is lengthyand may require several steps. Two examples on how this may beaccomplished are:

Example 1

Step 1: SWCNT+Li/NH₃→Li intercalated SWCNT

Step 2: Li intercalated SWCNT+X—R—NH-Fmoc SWCNT-R—NH-Fmoc+LiX (X=Br, I)

Step 3: Hydrolyzation SWCNT−R—NH-Fmoc+piperidine→SWCNT-R-NH₂

Step 4: SWCNT−R—NH₂+Epoxy resin SWCNT functionalized resins

The origin of step 1 and similarly of step 2 can be found in thefollowing paper: Liang et al., NanoLetters, 4, 1257 (2004).

Example 2

Step 1: SWCNT+HOOC—R—COO—OOC—R—COOH+heat SWCNT−R—COOH+CO₂

Step 2: SWCNT−R—COOH+epoxy resin SWCNT−R—COO—CH₂—CH(OH)—CH₂—.(esterification)

Steps 1 and 2 originate from work by Billups et al., Org. Lett., 5, 1471(2003) and is not efficient but has been demonstrated by Margrave etal., Nanolett., 3, 1107 (2003).

Multi-step functionalization of neutral CNT works but it is timeconsuming and costly. The control over functionalization degree remainsto be demonstrated. In addition, the effect of the chain length bearingthe functional groups on the overall property of the composites is notknown. Chemical functionalization is more costly than the productioncost of CNT, especially for SWCNT.

SUMMARY OF THE INVENTION

There is provided herein materials and methods for modifications ofCNTs. In an embodiment of the invention the nucleophilic character ofnegatively charged CNT is exploited to provide more efficient, moreversatile and more control over the composite properties. This isachieved, in one embodiment, by priming the surface of the CNTs toinduce negative charges thereon. In another embodiment it is achieved byreducing CNTs directly with radical anions produced by electron transferfrom alkali metal to the acceptor molecules such as naphthalene andbenzophenone.

BRIEF REFERENCE TO THE DRAWINGS

Figure A of the accompanying drawings is a schematic drawing of a priorart process described by Matrab et. al. (Atom transfer radicalpolymerization (ATRP) initiated by aryl diazonium salts: a new route forsurface modification of multiwalled carbon nanotubes by tethered polymerchains, Tarik Matrab et al. Colloids and Surfaces A: Physicochem. Eng.Aspects 287 (2006) 217-221);

FIG. 1 is a schematic drawing of the process of nucleophilic attack ofCNT employed in one aspect of the present invention;

FIG. 2 is a diagrammatic illustration of a general procedure to preparereduced CNT (Penicaud's method) useful in preparing starting materialsfor the present invention;

FIG. 3 is a diagrammatic presentation of an altenative procedure toprepare reduced CNT useful in preparing starting materials for thepresent invention;

FIG. 4 is a diagrammatic illustration of direct attachment of reducedCNT to epoxide functional groups in accordance with an embodiment of theinvention;

FIG. 5 is a diagrammatic illustration of attachment of functionalizedCNT to epoxy resins through base catalyzed ring opening, in accordancewith another embodiment of the invention;

FIG. 6 is a diagrammatic illustration of a process of functionalizingCNT with a chain bearing a hydroxyl function, for subsequent use in anembodiment of the invention;

FIG. 7 is a diagrammatic illustration of an alternative process forfunctionalizing CNT with a chain bearing a hydroxyl function;

FIG. 8 is a diagrammatic illustration of a process of using negativelycharged CNT as an initiator of polymerization, to make materials inaccordance with the invention;

FIG. 9 is a diagrammatic illustration of a process of graftpolymerization onto CNT followed by reaction thereof with epoxidemoieties; and

FIG. 10 is a schematic illustration of a process of reacting negativelycharged (reduced) CNT with various functional groups in accordance withthe invention.

DESCRIPTION OF THE INVENTION

Nucleophilic attacks of CNT can be employed, for example as schematizedin FIG. 1 of the accompanying drawings. CNT are primed to inducenegative charges thereon, indicated by Nu− on

FIG. 1. The CNT can be primed in one of two ways. In the first method,neutral CNT can be made to react with appropriate reagents to arrive atfunctionalized CNT in which negative charges are present. A second,presently preferred, method is to use reduced CNT. Reduced CNT can beprepared according to the method developed by Penicaud et al. (PCTapplication: WO 2005/073127: JACS 127, 8-9 (2005)). This methodeffectively charges up the CNT or its surroundings negatively by usingradical anions. The reduced tubes thus acquire nucleophilic character.The general procedure to prepare reduced CNT with Penicaud's method isdiagrammatically illustrated in accompanying FIG. 2.

Penicaud's procedure is carried out in THF and it has the advantage ofdispersing the CNT at the single tube level or at least in very smallbundles because of electrostatic repulsion between adjacent CNT. In someinstances it will be desirable to use different solvents which are notpractical and/or desirable for use with Penicaud's method. For example,toluene, ether, hexane and/or THF (tetrahydrofuran) can be employed whenan approach is required which avoids the formation of naphthalene alkalicomplexes. In some instances it will be desirable to form alkalibenzophenone complexes. Such complexes can be stabilized in toluene. Inthis example the electron donor is a benzophenone radical anion. Anexample of such a method is illustrated diagrammatically in accompanyingFIG. 3.

Negatively charged (reduced) CNT can react with various functionalgroups as shown in FIG. 10.

Examples, depicting certain embodiments of methods to attach CNTs toepoxide functional group containing resins, are provided below:

Method 1: Direct attachment of reduced CNT to epoxide functional groups.

As used in this example, the term “direct” indicates that the epoxidegroups react directly with the partially negative carbon atoms makingthe side walls of CNT. The partial negative charge on each carbon atomforming the CNT results from electron transfer from the radical anions.Thus, there is no need for a “spacer” between the CNT wall and the epoxyresin backbone. An embodiment of this approach is depicted in FIG. 4.

It will be understood that the epoxide functional groups may be on anymolecule with properties suited for its intended application. Forexample in the structure Δ_(R) (or, more generally “F₁-R-F₂” where “F₁”and “F₂” are the functional groups active in the depicted reaction), “R”can be alkyl, such as C₁-C₁₀₀₀, C₅-C₅₀₀, C₈-C₁₀₀, C₁₅-C₅₀. R may bealkane, alkene, alkyne, linear or branched, or aromatic. It may includeother functional groups and heteroatoms which do not significantlyinterfere with the desired reaction by F₁ and F₂.

In a typical experiment, 50 mg of SWCNTs (4.16 mmol of carbon) wasground with a mortar using a few drops of THF and then sonicated(Branson, model 5510 sonication bath) in 60 ml of dry THF until awell-dispersed suspension formed. Small pieces of sodium metal andnaphthalene solid were added into the suspension through which N₂ wasbubbled. The mixture was stirred overnight at room temperature and wasvisually characterized by its green color. Henceforth, this mixture willbe called the green solution. The reduced SWCNT were separated from thegreen solution by centrifugation under inert atmosphere. The reducedSWCNT were washed under inert atmosphere with dry THF twice to removeexcess of sodium naphthalene salts and free naphthalene. The paste (orprecipitate) of reduced SWCNTs was re-suspended in dry THF and mixed upwith de-oxygenated (by sparging with Ar or N₂) epoxy resin MY0510(triglycidyl-p-aminophenol resin, obtainable from Huntsman Chemical)under strong mechanical or magnetic stirring and under nitrogen or argonflow. The SWCNTs loading can range from 0 up to 10 wt % or higher. Aftermixing, the THF solvent was evaporated by sparging with strong Ar or N₂flow. A very important aspect of this method is that the amount of crosslinking and hence the final viscosity can be controlled by controllingthe amount of oxidizing and hydrolyzing agents into the sample, which isdone by sparging with wet air rather than inert atmosphere. Hence, thefinal product can be a viscous liquid, a rubbery solid or a soliddepending on the sparging conditions used. This method affords thepossibility of eliminating curing agents. Good control is exercised overthe final product mixture.

It will be appreciated by those skilled in the art that these methodscan also be employed to link CNTs to molecules having other functionalgroups instead of (or in addition to) epoxides. For example alkylhalides such as 1-Bromo(or Iodo)dodecane, 1-Bromoalcohol,1-Bromoethylene amine, Bromo-carboxylic acid, Bromo-carboxylate ester,succinic anhydride, Epibromoanhydride, DMSO, and all kind of currentlyavailable commercial epoxy resins

Here, the sample was prepared by sparging air into the sample. Themoisture and oxygen from air effectively neutralized (oxidized) thereduced SWCNT and hydrolyzed the nucleophilic centers and thus terminatefurther cross-linking

In an alternative finishing procedure, nitrogen and air weresubsequently used to obtain a rubbery material. In another alternative,the sample was prepared absolutely under inert atmosphere and the finalproduct was a solid.

Method 2: Attachment of functionalized CNT to epoxy resins through basecatalyzed ring opening.

The general idea is to first functionalize neutral CNT with chainsbearing hydroxyl functional groups which are then deprotonated withalkali metal to form alkoxides or aryloxides. Alkoxides and aryloxidesare known to react readily with epoxide moieties. The difference withmethod 1 is that here the CNT are separated from the epoxy resinbackbone by a spacer of fixed length.

As described with respect to Method 1, above, “R” may be any number ofthings in the structure CNT—R—OH. The following illustrative examplevalidates the above method.

A suspension of 1.145 g (95.4 mmol) of SWCNT in 250 ml of 1,2-ODCB and120 ml of acetonitrile was mixed with 2.3 equivalents of 4-aminobenzylalcohol (27 g, 219.2 mmol) and 3 equivalents of isoamyl nitrite (33.5 g,38.2 ml). The mixture was heated up to 70° C. over a weekend. Aftercooling down to around 50° C., the mixture was diluted with DMF andfiltrated. The precipitate was washed with hot DMF and methanol a fewtimes and dried. The scheme of this procedure is given in accompanyingFIG. 6, and was first reported (Chem. Mat., 13, 3823 (2001) anddescribed in the patent literature (US2005/0207963, WO 02/060812, GB2412370) by Tour et al.

The resulting dry material, SWNT−C₆H₄—CH₂OH, was re-dispersed in dry THFby grinding and sonication technique, and a slight excess of Na wasadded. The mixture was stirred for a day. The mixture was added to apreviously de-oxygenated sample of MY0510 and stirred vigorously. Thequantity of resin was adjusted based on the requirement of the SWCNTloading by weight (0.2, 0.4% etc). The mixture was stirred for a day,then sparged with wet air or wet nitrogen to terminate the cross linkingprocess and to evaporate the main part of the solvent (if the mixturewas kept under nitrogen and barged with dry nitrogen, the resin willeventually solidify). Again, the procedure offers some control over thedegree of cross-linking required. The remaining trace of solvent wascompletely removed in vacuum oven at 60° C. overnight.

It will be understood by those skilled in the art that this reaction isnot limited to CNTs with —OH groups. For example, thiol functionalizedCNTs may be used.

After nine months of ageing, a resin formulation prepared with thisprocedure retains very good dispersion and has not apparently changedover the months.

Hydroxyl functionalized SWCNT can also be prepared through the schemeillustrated diagrammatically in FIG. 7 and which is somewhat similar to:(NanoLett., 4, 1257 (2004)) and described in the patent literature byBillups et al., (WO 2005/090233).

Method 3: Block functionalization of reduced CNT using lengthcontrollable monomer, oligomer or polymers as spacer.

This is based on the recognition that the negative charges in reducedCNT act as anionic initiator for polymerization of monomer such asstyrene or methyl methacrylate (MMA) on CNT through the process known asgrafting from, followed by termination through ring opening of theepoxide moieties provided by the epoxy resins. The general scheme withstyrene is depicted in accompanying FIG. 8.

The following procedure validates the above concept.

8 ml of styrene and 8.5 g of MY0510 resin were added to a suspension ofnegatively charged SWCNTs obtained from a green solution bycentrifugation and washing technique (see method 1). The mixture wasshaken vigorously and sonicated, then further mixed on a Vortex-mixerfor a few hours. The mixture was shaken for another two days, thendiluted with THF. After centrifugation, the precipitate was washed withTHF, CHCl₃ and THF a few times through a sonication-centrifugationcycle.

Method 4: Bi-functionalization of reduced CNT

This can be considered as a two-step process. In the first step,polystyrene or PMMA is grafted from previously prepared reduced CNT.This produces polymer grafted CNT. In a second step, the polymer graftedCNT are reduced again and made to react with the epoxide moieties ofepoxy resins. The second step is analogous to Method 1 described aboveexcept that the CNT used are polymer grafted. The overall processproduces CNT with two independent functionalities, hence the termbi-functionalization. An example of the whole process in which styreneis used is depicted in accompanying FIG. 9.

The method described above provides materials with 1) better solubilityin common solvents, 2) better dispersion properties in various epoxyformulations, and 3) more handles for property adjustment for compositeformulations.

It will be understood by those skilled in the art that a wide range ofmonomers can be employed. Preferably, the monomer selected permits theeasy formation of a radical through various initiation processes(photolysis, thermolysis). In some instances it will be desirable to useone or more of styrene, olefins, lactone and lactide as the selectedgrafting polymer.

There is disclosed herein:

-   -   1) Methods for the covalent attachment of CNT to epoxy resins        through nucleophilic reactions with epoxide moieties.    -   2) Methods for the covalent attachment of reduced CNT to epoxy        resins.    -   3) Methods for the covalent attachment of alkoxide        functionalized CNT to epoxy resins.    -   4) A method for the reduction of CNT through electron transfer        from benzophenone alkali salts in toluene    -   5) Methods for direct, spacer-free, covalent attachment of CNT        to epoxy resins.    -   6) Methods for indirect, with fixed and variable length spacers,        covalent attachment of CNT to epoxy resins    -   7) Methods for the preparation of CNT functionalized with two        independent functional chains.    -   8) Methods for the preparation of CNT functionalized with two        independent functional chains in which one chain is polystyrene        or any other polymers dependent on the applications and the        other an epoxy resin (monomer or polymer)    -   9) Methods for the preparation of CNT functionalized with two        independent functional chains in which one chain is PMMA or any        other polymers dependent on the applications and the other an        epoxy resin (monomer or polymer).

What is claimed is:
 1. A process of chemically attaching compounds tocarbon nanotubes (CNTs) which comprises: functionalizing the CNTs toacquire anionic character via a negative charges, and subjecting thefunctionalized CNTs to nucleophilic substitution reaction with acompound carrying a functional group which reacts with the negativelycharged CNT, whereby the compound chemically bonds to the CNT.
 2. Theprocess of claim 1 wherein the functionalization of the CNT is performedby reducing the CNT to produce negatively charged CNT.
 3. The process ofclaim 2 wherein the CNT reduction has been accomplished by negativelycharging the CNT by use of radical anions.
 4. The process of claim 3wherein the reduction is conducted with alkali metal naphthalenecomplex.
 5. The process of claim 3 wherein the reduction is conductedwith alkali metal benzophenone complex.
 6. The process of claim 1wherein the compound carrying the functional group is an epoxy compound,whereby the epoxy group opens to form a C—C covalent bond to the sidewall of the CNT.
 7. The process of claim 1 wherein the compound carryingthe functional group is an alkyl halide, an alkylalcohol halide, acarboxylic acid halide, a halo-carboxylate ester, a succinic anhydride,or an epihaloanhydride.
 8. The process of claim 1 wherein thefunctionalization of the CNT is performed by attaching linkages to aside wall of neutral CNT and transforming the linkages to anions.
 9. Theprocess of claim 8 wherein the linkages are carrying alcohol groups,phenol groups, thiol groups, or secondary amine groups.
 10. The processof claim 8 wherein the anionic linkages are reacted with an epoxycompound.
 11. The process of claim 8 wherein the anionic linkages arereacted in one step with one of an epoxide, styrene ormethylmethacrylate to produce a CNT grafted with epoxide, polystyrene orpolymethylmethacrylate respectively, followed by a step of furtherreduction of the grafted CNT to produce a grafted, reduced CNT, and thenin a further step the grafted, reduced CNT is reacted with another ofepoxide, polystyrene or methylmethacrylate, to produce abi-functionalized CNT.